This invention relates to membrane switches having silver conductors, and particularly to an improved conductor construction that minimizes silver migration in the region at which electrical connectors are affixed to the switch to enable connection between the conductors and external devices.
A membrane switch typically consists of a bottom layer and a top layer spaced from, but selectively movable into contact with, said bottom layer. Conductors of silver are painted, printed or silk-screened onto the bottom layer and the facing surface of the top layer, silver being employed because of its high electrical conductivity. However, the use of silver presents a serious drawback due to a phenomenon known as silver migration--i.e., the tendency of the silver particles, in a high humidity atmosphere, to migrate from the anode to the cathode, ultimately forming a high impedance short between the separate conductors. A more detailed description of the problem of silver migration is set forth in Larson U.S. Pat. No. 4,243,852. In order to inhibit silver migration, the silver conductors of a membrane switch are often over-coated with a layer of non-migrating, conductive material, such as graphite, in the regions most susceptible to silver migration.
A typical membrane switch includes a tail portion integral with and extending outwardly from the edge of one or both of the conductor-carrying layers, on which tail or tails the individual conductor leads are carried for connection to external circuitry. In order to make the connection to external circuitry, it is the usual practice to attach an electrical connector to each of the conductive leads at or near the outward end of the tail. However, the normal piercing or crimping operation by which a connector is attached to a conductive lead on the tail results in a break in the bond between the silver conductor and the protective over-coating, such that, in the region of the break, the exposed silver is subject to silver migration.
Therefore it is an object of this invention to provide an improved membrane switch in which silver migration at the area of the electrical connectors, normally attached to the conductive leads at or near the end of a tail extending from the switch, is minimized. Other purposes will be evident from the specification, drawings and claims.